1. Field of the Invention
This invention relates to directed infrared countermeasure (DIRCM) systems, and more particularly to a DIRCM system for use with commercial aircraft to effectively counter MANPADS and more advanced threats.
2. Description of the Related Art
The proliferation of shoulder-launched missiles known as MANPADS for “Man-Portable Air-Defense System” and their availability to terrorists present a real threat to military aircraft and particularly commercial aircraft. Estimates of the number of attacks on commercial aircraft vary, running as high as 43 hits on civilian aircraft—with 30 of these resulting in aircraft kills and the loss of nearly 1,000 lives—since the 1970s. More than half a million MANPADS have been delivered worldwide, and many of these are still operational. These missiles currently use infrared (IR) seekers to track and lock-on to the aircraft. The missiles typically have a range of 5-8 km and can reach an altitude of approximately 12,000 ft. Historically, countermeasures range from active IR jamming to flares and chaff.
As illustrated in FIGS. 1 and 2, a terrorist 10 holds a MANPADS 12 on his or her shoulder, points it at the aircraft 14 and launches the missile 16. A typical missile 16 will typically progress through the eject, boost, sustain and possibly post-burn stages before impacting the aircraft. The missile's IR seeker 18 tracks IR energy emitted by the aircraft 14. The seeker processes the infrared scene containing the target and generates target tracking information 22 that guides the missile 16 enabling the seeker to track hot targets like aircraft 14. The aircraft's DIRCM system 24, suitably mounted in a “blister pack” near the rear of the aircraft, must detect, verify, track and then emit a modulated laser beam 26 or eject flares that produce a false signature 28 to jam the missile's IR seeker. The purpose of either approach is to generate a false target with a “miss distance” from the aircraft. The DIRCM system will typically try to detect the missile at ejection based on the eject motor's impulse signature, verify the threat and track the heat plume 30. The DIRCM system is particularly stressed when the shot is taken from close range such as might be the case on take-off or landing or when multiple simultaneous shots are taken at the aircraft, this later case being taught to terrorists undergoing formal training.
An effective DIRCM system must be able to detect and verify the threat and jam the IR seeker with low false-alarm rates during burn and post burn out of the missile's flight motor. False alarms should occur no more than once every 100 take-offs or landings, or 17 hours of operation, whichever is the lower. To be effective against short-range shots and multiple closely timed shots, the DIRCM system must be able to respond very quickly, less than 1-2 seconds, to engage and neutralize the threat. The system should have a probability of success of at least 90% against multiple MANPADS launches, or 80%-plus against two missiles with simultaneous impact times. The DIRCM system should have the capability to extend to more sophisticated threats from missiles outfitted with both IR and UV seekers.
DIRCM systems for use in commercial aviation must satisfy a number of other criteria without sacrificing effectiveness. The cost of any system must be affordable including the unit, certification, installation, operation and maintenance. The system technology must be exportable. The system should present minimal additional aerodynamic drag, be light weight and have a relatively low power budget to avoid increasing fuel costs. The system should be capable of transmitting the modulated laser beam at high power levels in order to effectively jam the missile from maintaining lock on the large cross-sections of commercial aircraft. Furthermore, the system should have no negative psychological impact on the flying public and post no threat to the environment, passengers or flight crew.
Northrop Grumman's Large Aircraft Infrared Countermeasures (LAIRCM) uses AN/AAR-54(V) Missile Warning System (MWS) sensors, operating at ultraviolet wavelengths to detect the weapon's exhaust plume, to provide initial alerting. Each sensor covers a 120° field of view and has a high resolution to discriminate threats from clutter. The MWS supplies its measurements to the system processor, which evaluates the signals and—if they are determined to represent a hostile missile—declares the target as a threat. In response, the “gimbal” or “jam head” containing the fine-track sensor (FTS) and jammer slews to the appropriate direction. The FTS locks on to and tracks the threat, and continuously slews the gimbal so that it remains pointed at the incoming missile. The transmitter then employs a modulated beam of infrared energy to jam the weapon's guidance signal. This entire process typically takes 1-2 seconds for the easiest cases of close and mid range shots where the launch motor impulse is detected and boost sustain motor signature is highest. This time increases when the launch pulse is weak or masked or the motor has burned out. See “David versus Goliath”, International Defense Review, Apr. 1, 2004, Mark Hewish and Joris Janssen Lok.
The LAIRCM is built on the same platform as its predecessor Nemesis but uses a laser instead of a flashlamp. As a result the FTS is very heavy and does not respond quickly. Therefore the LAIRCM and other similar systems must verify the threat before slewing the gimbal to initiate tracking. The LAIRCM uses free-space optics to optically couple the laser output to the gimbal. The optical path has very high losses, which reduces the output power of the modulated laser. Furthermore, the air-glass interfaces of the free-space components are highly susceptible to contamination and damage, which reduces the reliability of the system. In addition, the LAIRCM can not support an additional UV laser in the same gimbal to counter more advanced threats because the internal optics and transmit ports do not transmit UV. Furthermore, LAIRCM is very expensive, contains classified technology, is very heavy, consumes a lot of power and was not designed with high production rates in mind. Production rates are far below those requested in the commercial arena and no war surge capacity is known to exist.
BAE Systems has demonstrated a DIRCM system that is similar to the LAIRCM except that it uses optical fiber to couple the laser to the gimbal (BAE Systems, Nashua N.H. IRIS Paper 2001CMC02x Infrared-Transmitting Fibers for Advanced IRCM Systems Demonstrations May 2001). The implementation of an “All-Fiber Path” was intended to improve output power and reliability. However, to achieve the necessary gimbal dual-axis rotation, three discrete fiber segments are coupled to each other using custom-made fiber optic rotary joints, which are capable of 360° rotation. The two short sections of fiber cable used inside the head are of larger core diameters equal to 200 and 250 μm, respectively, to prevent potential loss due to rotation misalignment of the joints.
Using multiple fiber segments inside the gimbal resulted in the elimination of several actuated mirrors and servo loops which reduces complexity and could potentially enhance system reliability. However, piping the fiber through the two gimbal axes (Roll and Nod) mandated the use of non-continuous fiber segments coupled optically with optical rotary joints. The optical rotary joints have insertion and extraction losses at each interface between the fibers on the input and output, optical elements (input and output face) and at each air gap (minimum of three air spaces and six AR coatings per rotary joint). Each air gap is subject to contamination and as experienced during the demonstration damage to the AR coatings.
In the demonstration by BAE approximately 3.3 W of laser power was used in the 3-5 μm wavelength region. It was found that even at the modest power levels used in the BAE demonstration the high peak optical power from the laser caused damage to the AR coatings; however, the fibers were undamaged and continued to transmit the laser power. The rotary joints also introduce additional loading on the gimbal torquers resulting in reduced slew rates and increased settling times adversely impacting system performance in multiple short shot engagements. The rotary joint limits any attempt to significantly downsize the gimbal to reduce aerodynamic drag due to mechanical limitations inherent in the design of the rotary joint, fiber fittings and fiber protective sheathing which limit fiber bend radius on each side of he fiber optic rotary joint.
Similar to the LAIRCM system, the BAE system must first detect and verify the threat before slewing the tracker to initiate tracking. The use of the segmented fibers and optic rotary joint eliminates some free-space components but still suffers from losses and damage associated with free-space optics. The use of optic rotary joints would, at a minimum, make it very difficult to incorporate a UV fiber and transmitter in the same gimbal.
Most of the available DIRCM systems use 1-color MWS to detect and verify the threat and 1-color pointer-trackers to track the threat once verified. To improve tracking capability some systems, e.g. TADIRCM Development Program PMA-272, use 2-color pointer-trackers where 2-color simply refers to two separate bands in the mid-IR. To improve the detection range and reduce the false-alarm rate, system providers are developing 2-color MWS.
Because these DIRCM systems are all variants of mechanical and algorithmic platforms initially built to counter surface-to-air and air-to-air missiles launched against military aircraft using flashlamps and not lasers they do not adequately address the threat posed by MANPADS to military and particular commercial aircraft. DIRCM systems with faster response times, higher power and greater reliability are needed. These systems should also be capable of incorporating a UV/Visible laser in the same gimbal to counter currently fielded 4th generation threats like Stinger RMP and those in development like the system described in Patent RU 2160453 C2 Leningrad Optical-Mechanical Company (LOMO), Optical-Electronic Seeker. The system must be low-cost, exportable and highly effective.